(1) Field of the Invention
The present invention relates to an output buffer circuit having three-output states including a high level, a low level and a high impedance.
(2) Description of the Prior Art
The prior art will be explained by referring to the following drawings: FIG. 1 designates a block diagram illustrating a conventional bipolar 4K-bit Programmable Read-Only Memory PROM device; FIG. 2 is a partial and detailed circuit diagram of the PROM of FIG. 1; FIG. 3 is a circuit diagram which is equivalent to a transistor cell used in the memory cell array in FIGS. 1 and 2; and FIG. 4 is a circuit diagram showing an output buffer employed in the PROM shown in FIGS. 1 and 2.
As shown in FIG. 1, the conventional bipolar 4K-bit PROM device comprises a memory cell array 2 composed of 4096 memory cells disposed in a matrix of 64 rows and 64 columns. Each of the memory cells is composed, for example, of an open-base type transistor which is equivalent to a series connection of two diodes of opposite polarities. The writing-in of information into one of the memory cells is effected by forcing one of the PN junctions into a short-circuit status with a large writing-in current. A word address buffer or word address inverters 4 amplify and invert input word address signals A.sub.0 through A.sub.5, and word address decoder/driver 6 decodes the word address signals from the word address buffer 4 and applies the decoded word address signals to the memory cell array 2. Bit address buffer or bit address inverters 8 amplify and invert input bit address signals A.sub.6 through A.sub.9, and apply the output bit address signals to bit address decoder 10 connected to multiplexer 14 and bit address decoder 12 connected to program circuit 20. The decoder 10 decodes the output bit address signals from the bit address buffer 8 and applies the decoded bit address signals (bit selection signals for reading-out) to the multiplexer 14. The multiplexer 14 reads out information from the memory cells connected between the word line selected by the above-mentioned word address decoder/driver 6 and the bit lines selected by the bit address decoder 10. The output buffer 16 amplifies the read-out signals from the multiplexer 14, and applies the amplified read-out signals to output terminals OP.sub.1 through OP.sub.4. The output buffer 16 is enabled by chip enable circuit 18 which receives an input chip enable signal. The decoder 12 decodes the output bit address signals from the bit address buffer 8 and applies the decoded bit address signals (bit address signals for writing-in) to the program circuit 20. The program circuit 20 writes into information to the memory cells which are connected between the word line selected by the word address decoder/driver 6 and the bit lines selected by the bit address decoder 12.
The reading out of information from the above-mentioned PROM is effected by applying the 6 single bit word address signals A.sub.0 through A.sub.5 to the word address buffer 4, the 4 single bit address signals A.sub.6 through A.sub.9 to the bit address buffer 8, and a low level signal to the chip enable circuit 18. The 4 bits of information from the selected memory cells appear at the output terminals OP.sub.1 through OP.sub.4, via the multiplexer 14 and the output buffer 16.
The writing-in of information to the PROM is effected by applying the word address signals A.sub.0 through A.sub.5 to the word address buffer 4, the bit address signals A.sub.6 through A.sub.9 to the bit address buffer 8, and a high level signal to the chip enable circuit 18. The 4 bit of data to be written-in applied to the output terminals OP.sub.1 through OP.sub.4 of the PROM device. In this case, the output buffer 16 is cut off from the output terminals OP.sub.1 through OP.sub.4 because of the high level signal applied to the chip enable circuit 18. Therefore, the data signals to be written-in pass from the output terminal to the program circuit 20, and the writing-in of information corresponding to the data signals into the selected memory cells is effected.
The writing-in operation will be described more in detail. As shown in FIG. 2, for example, the PROM comprises memory cells C.sub.00, C.sub.01, - - - , each consisting of a bipolar transistor whose base is shown not connected and being connected to each of the points at which word lines W.sub.0, W.sub.1, - - - cross bit lines B.sub.0, B.sub.1, - - - , respectively. Writing into the PROM is carried out as follows: a large writing-in current of about 120mA is applied to a terminal which serves as a read output terminal and as a program (or write) terminal; one of the bit lines e.g. bit line B.sub.0 is selected by the program circuit 20 and to the bit line selected is applied the writing-in current; one of word lines e.g. word line W.sub.1 is selected by the word address decoder/driver 6 and the word line selected is made the low level; and the writing-in current is applied to the cell C.sub.10. When writing is carried out as described above, the base-emitter connection of cell C.sub.10 is broken down to short-circuit state and only the base-collector connection thereof becomes equivalent to a forward direction or bias diode. In the case of cells to which such writing is not applied, the base-emitter connection of each of them serves as a reverse polarity or bias diode in the circuit and current is therefore not allowed to flow even if a read voltage (Vcc) of about 5 V is applied between the bit and word lines. In the case of cells to which writing-in current is applied, read-out current is allowed to flow. Responsive to such an ON and OFF of the read-out current, information "1" and "0" is provided as output data.
With respect to the write transistor cell C.sub.10, the write transistor can be designated by a series connection of two diodes D.sub.CB and D.sub.BE as shown in FIG. 3. The respective anodes of the diodes D.sub.CB and D.sub.BE are connected together and to correspond to the base of the write transistor. The cathode of the diode D.sub.CB corresponds to the collector thereof and the cathode of the diode D.sub.BE corresponds to the emitter thereof. In order for the write operation to be carried out in the PROM, the diode D.sub.BE is caused to break down and to be short circuited.
Reading out is carried out as follows: read voltage (V.sub.CC) is applied to a bit line selected by the multiplexer 14; a word line is grounded by the word address decoder/driver 6 through a transistor Q; and potential appearing at the bit line is applied through the multiplexer 14 to an output buffer 16. This potential appearing at the bit line becomes the high level when no writing-in current is applied to the cell and of low level when writing-in current has been applied to the cell. These levels serve as read outputs.
The output buffer 16 is an amplifier which generates the read output responsive to the potential appearing at the bit line at the time of reading out. The output section of output buffer 16 is of a three-state type that does not cause any trouble by interrupting the high writing-in current which is applied to its output terminal at the time of programming or writing-in. As shown in FIG. 4, this output buffer 16 comprises an input stage transistor Q.sub.1, a phase splitting transistor Q.sub.2, output stage transistors Q.sub.3 and Q.sub.4, resistors R.sub.1 -R.sub.4, and a diode D, and the output stage is of a three-output type like a totem pole. In place of the transistor Q.sub.3 and the diode D, a darlington pair of transistors may also be used. Ti represents an input terminal connected to the multiplexer 14. If the potential appearing at the bit line is the low level, current is not allowed to flow to the collector side but is allowed to flow on the emitter side of transistor Q.sub.1, so that the transistor Q.sub.2 is turned OFF. Transistor Q.sub.3 is turned ON while the transistor Q.sub.4 is turned OFF and output terminal T.sub.0 is made high. However, if the potential appearing at the bit line is the high level, current is allowed to flow to the collector side of transistor Q.sub.1, so that the transistor Q.sub.2 is turned ON, while transistor Q.sub.3 is turned OFF, transistor Q.sub.4 is turned ON and output terminal T.sub.0 is made low. When output of inverter I is made low, the base and collector of transistor Q.sub.2 become the low level through diodes D.sub.1 and D.sub.2, so that transistors Q.sub.2, Q.sub.3 and Q.sub.4 are turned OFF and the output terminal T.sub.0 is turned to the high impedance state. In FIG. 4, the diodes D.sub.1 and D.sub. 2 may be replaced by PNP-transistors whose bases are connected to the output of the inverter I, collectors are connected to the base and collector of the phase splitting transistor Q.sub.2 and emitters are connected to the ground. Even if a writing-in current is applied to the terminal T.sub.0 which has been turned to such a high impedance state, the high voltage is not clamped by the output stage elements of output buffer 16 to ground or the Vcc level.
In order to put the output buffer in the impedance state, it is necessary that signal S be applied to the input terminal of inverter I to cause the output of the inverter to be the low level. In addition, when the writing mode is changed to the reading mode, it is necessary that signal S be changed to make inverter I output the high level.
Further, even in this three-output-state circuit, both of output stage transistors Q.sub.3 and Q.sub.4 are turned ON which wastes a large amount of current, thus causing output stage transistors to breakdown. When the amplitude of input signal e.g. the potential appearing at the bit line is made small intending to achieve a high speed operation and at the same time the power source voltage (Vcc) changes to a higher level, for example, the input stage transistor Q.sub.1 allows current to flow to both the emitter and collector sides thereof, so that the phase splitting transistor Q.sub.2 is incompletely turned ON and as a consequence both of transistors Q.sub.3 and Q.sub.4 are turned ON, thus causing output stage transistors to be broken more easily.